Due to the limited radio spectrum allocated by governmental bodies, various techniques have been employed to allow multiple users to access the allocated radio spectrum. One type of multiple access technique is known as Code Division Multiple Access (CDMA). In systems which operate using CDMA techniques, an information data stream to be transmitted is modulated by a data sequence with a much higher data rate, referred to as a “signature sequence.” The signature sequence usually comprises N bits, wherein each of the N bits is denoted as a “chip.”
A well-known source of degradation common to all known wireless multple access systems, particularly in terrestrial environments, is known as “multipath fading.” In a multipath environment, the transmitted signal follows several propagation paths from a transmitter to a receiver, typically as a result of the signal reflecting off one or more objects before arriving at the receiver. Since the various propagation paths of the transmitted signal are of unequal lengths, several copies of the transmitted signal, referred to as “rays,” will arrive at the receiver with varying time delays. In a multipath fading channel, phase interference between different rays may cause severe fading and result in signal dropout or cancellation.
A mobile station in a CDMA system is typically equipped with a RAKE receiver for demodulating active channels (i.e., channels carrying control and/or user data) and for compensating for multipath delays. The RAKE receiver comprises a number of RAKE fingers and a combiner for combining the output of each of the RAKE fingers. When demodulating a multipath fading channel, each finger of the RAKE receiver must be synchronized with one of the diverse propagation paths of the channel. A RAKE receiver with L fingers is able to detect, at most, L copies of the transmitted signal, which are individually despread by the RAKE fingers according to the individual delays and are added coherently in the combiner. At the addition performed by the combiner, each despread output from a RAKE finger is multiplied with a complex weight. Typically, these weights can be set as the complex conjugate of the corresponding coefficient of the channel impulse response at the appropriate delays. For this, the channel impulse response must be estimated at the delays, a process which, for example, can be made by a separate algorithm in the digital signal processor. The resulting signal will thus comprise a collection of all the time delayed copies of the transmitted signal.
The relative time delays of the received rays must be determined in order to synchronize the various rays with the corresponding fingers of the RAKE receiver. Unfortunately, the number and magnitude of the time delays may change, e.g., due to the movement of the mobile station, i.e., variable distance and velocity relative to the transmitting base station and reflecting objects for users in motion. Also, movement of the mobile station may cause new channel paths to appear and old channel paths to disappear. Hence, the mobile station must continuously monitor the signals received along all propagation paths of an active channel in order to search for new, stronger channel paths. To perform this monitoring efficiently, the multipath time delays must be measured or estimated repeatedly in a fast and accurate manner. Typically, this is performed by a delay estimator.
The simplest approach to delay estimation (DE) is evaluating the impulse response of the channel over the whole range of the possible delays, or the delay spread, of the channel. The resulting complex delay profile (CDP) or power delay profile (PDP) may then be subjected to peak detection, and the peak locations are reported to the RAKE as the delay estimates. However, the processing and power consumption expense of frequently executing this path searching routine is usually prohibitive. Therefore, typical implementations use shortened search windows, reduced searcher resolution, and additional short sub-searchers to produce higher-resolution estimates of certain areas of the power delay profile.
A typical approach, in the case where several distinct multipath channels with different path structure need to be characterized, includes applying a delay estimation and subsequent channel estimation algorithm to each of these channels.
FIG. 1 illustrates one conventional approach to delay estimation. Pilot samples received over the air interface are provided to path searcher 101. Path searcher 101 computes instantaneous impulse estimates (complex or power) over a range of delays that constitute a significant fraction of the maximal delay spread allowed by the system. The complex value or power value for a given delay value is estimated, e.g., by correlating the received data for pilot symbols with an appropriately delayed copy of the spreading sequence. The path searcher is employed mainly for detecting the existence of paths and its output resolution may be lower than that required by the RAKE.
The tuning fingers 103 receive the coarse power delay profile information provided by the path searcher 101 and produce a high-resolution instantaneous complex delay profile or power delay profile over one or several narrow delay windows. Path resolution, tracking and reporting block 105 extracts physical path location information from the path searcher 101 and the tuning fingers 103, and presents delay estimates consistently to subsequent RAKE receiver stages. Unchanging assignment of paths to RAKE fingers is necessary to support filtering of power and interference estimation for each finger. The degree of complexity of the block 105 can vary significantly depending on system parameters, ranging from simple peak detection to sophisticated deconvolution and filtering.
Scheduling and window placement block 107 determines the timing of activation of the path searcher 101 and the tuning fingers 103, and their window positions for each cycle. The timing may be fixed (periodic) or depends on signals derived from the environment, while the positioning usually depends on the location of previously detected paths.
To increase robustness of the delay estimation under various difficult channel conditions (low signal-to-interference ratio (SIR), wide delay spread, closely-spaced paths, etc.), averaging of memory can be added to the algorithms so that the delay estimation process operates across many channel fading cycles and is not significantly affected by the instantaneous fading realization.
Following delay estimation, channel estimation for the reported delays is performed by tuning despreaders to these delays and using the despread pilot symbols to deduce the complex path coefficient for a given delay. A variety of filtering or smoothing methods may be applied to these instantaneous estimates, in order to improve the quality of the channel estimates. These methods are well known in the art.
Regardless of the specific implementation, the complexity of the delay estimation process is significantly higher than that of the channel estimation operation. Similarly, the sensitivity of the delay estimation to low SIR conditions is significantly higher, causing rapid deterioration below a certain threshold, compared to the channel estimation process which degrades more gradually.
It can be appreciated that the quality of performance of a RAKE receiver is related to how well the channel delay estimator performs. The more accurate the estimates of signal path delays, the better the RAKE receiver will perform. An exemplary channel delay estimator 200 is illustrated in FIG. 2. The channel delay estimator 200 tests different delay versions of the received signal for correlation with a given spreading sequence. For each hypothesized delay, the degree of correlation determines whether the hypothesized delay represents an actual delay experienced by the received signal. To carry out this process, the exemplary channel delay estimator 200 has five “probing fingers”, each associated with one of the hypothesized delays: t0, t1, t2, t3, t4. These could, for example, be equally spaced with respect to one another, such as at 0, Δt, 2Δt, 3Δt, 4Δt, as illustrated in FIG. 2. As can be appreciated, there will always be some minimal amount of delay, so having t0=0 absolutely may not be physically possible. However, the delay associated with t0 may be used as a base offset, with each of the hypothesized delays reduced by the base offset amount, making it possible for t0=0 relative to the base offset. By making Δt small, it is possible to fine tune a delay estimate and track changes in the delay. The choice of five probing fingers in this example is merely for illustration: The number of probing fingers in any particular embodiment is a design choice that can be less than, equal to, or greater than five.
The received signal is supplied to a delay unit 201 that aligns the signal to be processed in accordance with the hypothesized delay. The (delayed) received signal is then passed through a matched filter or correlator 203. The matched filter 203 generates an estimate of the impulse response of the channel. This estimate is generally a complex-valued signal.
If the channel parameters are subject to fast changes, the estimates, made for each of the number N of time slots, may be summed non-coherently. This means that the absolute value of the complex signal is determined (block 205), and then summed with the values obtained for the signal during other time slots (summing block 207). Alternatively, if the channel parameters are subject to slow changes, then the channel estimates may be summed coherently, so that the absolute value block 205 would not be present. In other alternative embodiments, a combination of coherent and non-coherent averaging is also possible.
In either case, the result of summing blocks 207 for each position (0, Δt, 2Δt, 3Δt, 4Δt) are compared and the position having the highest summed value is selected, as illustrated in FIG. 2. The real-valued summed results for each signal position of the channel delay estimator are fed into a selector. The selector determines the position having the highest summed value. The parameters associated with this position, such as the estimated delay or impulse response, may be used by the RAKE receiver. For example, the position parameters may be used by a searcher to synchronize the RAKE receiver to different paths.
The fact that the channel is fading will prevent every time slot from contributing to the estimate of the delays. However, the variations of the channel in general are such that the fading process is much faster than the changes of the delays. Thus, if we assume merely for the sake of example that, on average, there are two equally strong paths with gain h1 and h2, two peaks will be built up over time in the cumulated sum over different time slots, so long as the delays are sufficiently well separated in time.
Conventional techniques for addressing multipath propagation may not correctly identify the delay of particular paths when the mutual difference in delay between multiple paths is small. These inaccurate delay estimations can cause serious degradation in the performance of the RAKE receiver.